This invention relates to an antenna, and in particular a dual mode antenna.
Mobile communications are becoming increasingly popular partly due to an increase in performance of communication devices and also to a decrease in their size.
One area, however, in which size reduction runs counter to performance is in antenna design where, generally, smaller antennas give reduced performance compared to larger antennas.
Consequently, to minimise space requirements in communication devices, for example a radiotelephone, the type of antenna typically used is a monopole antenna. The monopole antenna acts as a conductor placed counterpoised to a ground plane, typically a printed circuit board (PCB) in the radiotelephone. The conductor forms an image in the ground plane such that the resulting antenna pattern is a composite of the ‘real’ antenna (i.e. the monopole antenna) and the ‘image’ antenna (i.e. the image in the ground plane). Accordingly, a monopole antenna can be half the size of an equivalent dipole antenna providing that the groundplane makes up the other half of the dipole.
However, as communication devices reduce in size so typically does the ground plane associated with the communication device.
Consequently, if the electrical length of the ground plane becomes less than a quarter wave length of the ‘real’ antenna the antenna image is unable to match the current flow in the ‘real’ antenna. As a result, the efficiency of the antenna decreases such that either the bandwidth of the antenna is reduced, thereby reducing the performance of the communication device, or a larger ‘real’ antenna is required in order to compensate for the reduction in size of the ground plane, thereby increasing the size of the communication device.
One solution to this problem has been to use narrow bandwidth antennae that are tuned between different frequencies using a switched matching network. By moving the resonant frequency of an antenna between a range of frequencies, a wide bandwidth antenna can be simulated. However, this solution can increase the complexity and cost of an antenna arrangement and can also reduce performance.
It is desirable to increase the bandwidth of an antenna without a corresponding increase in the size or complexity of a communication device.